I. Field of the Invention
The present invention relates to optical scanning and reading equipment and, in particular, to systems for scanning a light beam across a target such as a bar code symbol.
II. Description of the Related Art
In a light beam scanner, a small spot of light is swept rapidly across a target such as a bar code symbol. After reflection from the target, a photoelectric converter such as a photo diode detects the reflected light and converts it to electronic signals representing features of the target. A bar code scanner is an important commercial application for beam scanners and is referred to herein as a typical example of a specific application for the present invention.
Various mechanisms exist in the prior art for sweeping (or scanning) a light beam across a target. One such prior art mechanism is shown in FIG. 9. In the mechanism of FIG. 9, a coil 145 wound on bobbin 146 is attached to a steel mounting frame 148. Steel rivets with heads 147a and 147b act as poles for coil 145. A scan element 149a has a fixed end 149b mounted to mount 149 which in turn is fastened to a scanner chassis. The scan element 149a is a laminated structure made from flexible copper clad printed circuit stock. The laminated structure at the fixed end has copper layer 133 laminated to flexible plastic film 135 (e.g. polyamide film) also laminated to copper layer 136 which is in turn affixed to mounting bracket 149. The flexible end 149c of the scan element includes copper layer 131 laminated to film layer 135 which in turn is laminated to copper layer 138. Flexible film layer 135 holds the fixed end 149b and the flexible end 149c of the scan element together.
The scan element shown in FIG. 9 includes an open area 137 that is formed by etching away a portion of copper between layer sections 138 and 136. The area immediately above area 137 is also been formed by etching the copper away between copper layer sections 133 and 131 and back filling this area with an elastomer fill 132. Mirror 134 is attached to copper layer 131 and magnet 140 is attached to copper layer 138. Magnet 140 causes mirror 134 to dither in the direction of double arrow 139 when acted upon by current introduced into coil 145. In the scan mechanism of FIG. 9, film layer 135 is a permanent part of the structure and is substantially thick (i.e., on the order of 0.030") imparting stiffness to the flexural characteristics of the device. Since film layer 135 acts as a stiffening layer, this structure must be made relatively large and heavy in order to achieve slow scan rates on the order of 20 Hz. Furthermore, the stiffening effect of plastic layer 135 reduces the Q of the system and acts as a damper so that many oscillations cannot be maintained by the introduction of only a single current pulse in coil 145. As described more fully below, one object of the present invention is to overcome this problem by providing a scan system wherein sufficient oscillations can be maintained by introduction of only a single current pulse in the drive coil or by application of some other momentary distortion force to the system.
Due to the relatively high rigidity of film layer 135, coil 145 must be large with an iron core having pole faces to sufficiently act upon magnet 140. Furthermore, mass manufacture of the device of FIG. 9 requires a significant number of steps and components. As described more fully below, it is a further object of the present invention to provide a scan system which does not require such a large coil and which can be mass manufactured more easily and cost efficiently than the mechanism of FIG. 9.
U.S. Pat. No. 4,593,186 entitled "Portable Laser Scanning System and Scanning Methods" discloses a portable laser scanning system for reading bar code symbols. The system includes means for generating and directing a laser beam, scanning means for scanning the laser beam across a symbol, sensor means for detecting light reflected from the symbol and generating a signal, signal processing means for processing the signal, decoding means for decoding the signal, manually actuatable trigger means for initiating each reading of the symbol, power supply means, and means for determining a successful decoding of each symbol and for non-manually terminating the reading of each symbol after a successful decode. Alternatively, the system includes means for determining that a symbol has not been successfully decoded and for non-manually terminating the reading of the symbol if the symbol has not been successfully decoded after a predetermined amount of time. Thus, in the system of the '186 patent, the deactuation of the scan system depends, at least in part, on a determination as to whether a symbol has been successfully decoded. In the present invention the determination as to whether the symbol has been successfully decoded or not is not necessary in order to properly and efficiently deactuate a scanning system. It is thus an object of the present invention to provide an improved and simpler sequence for actuating and deactuating the elements in a scan system which functions independently of any determination that the symbol has been successfully or unsuccessfully decoded. In doing so the benefits are significant, among these are: less battery energy needed, easier to use, fewer components needed, lower assembly costs, less space required to mount the scanner and quicker scan sequences.
The scan systems disclosed in the '186 patent also include motors that must be supplied power continuously during the entire scan sequence. To conserve power the motors are shut down in response to, for example, determinations that the symbol has or has not been successfully decoded. As will be explained more fully below, it is a further object of the present invention, to provide an actuation sequence that is more efficient than that described in the '186 patent, and which does not require continuous power to the scan element during the entire scan sequence.